Current color papers utilize three color layers comprised of a red light sensitive cyan layer, a green light sensitive magenta layer, and blue light sensitive yellow layer. In order to increase productivity of photofinishing operations, there is a strong desire to have color papers with increased overall sensitivity. In the color negative printing system, the sensitivity of the blue color record is usually considered to be the limiting factor. One common way to increase sensitivity is to use a larger emulsion grain size. However, a larger grain size causes a number of problems.
In particular, large grain emulsions typically lead to very dense dye clouds. This results in a lowering of dye covering power. In other words, more dye is necessary to achieve the same reflection density in a color photographic reflective print. Thus, the effect is to lower the efficiency of the entire system. To achieve adequate reflection density, more color forming coupler and more silver halide must then be used in the photographic element.
These effects are especially evident in areas of high dye density, that is, in the shoulder and Dmax (area of maximum density) regions. For instance, in the case of the blue sensitive layer, a low shoulder would lead to black areas going blue and yellow colors desaturating. Likewise, for a green sensitive layer, a low shoulder would give greenish blacks and desaturated magentas, while for a red sensitive layer, reddish blacks and desaturated cyans would result.
A further effect of using large grained emulsions is a deleterious loss of color purity. The formation of very tight dye clouds causes the hue of the dye to become muddy. This happens because the tails of the spectrophotometric curve for the dye will tend to become exaggerated, and the effective hue is broader than for a more diffuse dye cloud situation. In the case of yellow dyes, the hue becomes desaturated and shifted undesirably toward orange.
Another problem with the use of large grained emulsions is the tendency for the oxidized developer (Dox) to be formed in too high of a concentration to be used effectively by the color forming coupler. Thus, more of the Dox wanders from the color record in which it was formed into adjacent layers. Typically, color photographic elements contain Dox scavenging interlayers (otherwise known as anticolor-mixing layers) to prevent interlayer color contamination. In the case of large grained emulsions, the scavenging layer effectiveness must be increased with more materials to react with excess Dox and prevent it from reaching another color forming layer. This also results in materials being wasted, since no dye is formed as a result of this process.
In some instances, the very large grained emulsions produce a local concentration of Dox that is too high to be completely used by the appropriate coupler and/or scavenged by an anticolor-mixing agent. For instance, in the case of large grained emulsions used in the blue layer, this can result in the formation of magenta colored spots in the photographic print from reaction of Dox formed in the blue layer with magenta coupler situated in the green layer.
Another problem with using large grained emulsions in order to gain additional photographic speed is an undesirably large change in sensitivity with respect to the photographic element's level of hardness. This can be very deleterious because the photographic paper must be stored until completely hardened in order to assure product uniformity. This necessitates keeping a large inventory and creates undesirable additional costs for color photographic paper manufacture.
U.S. Pat. No. 4,040,829 describes a structure where a semi-diffusable coupler layer is coated on top of the topmost emulsion layer.
European Patent Application No. 0 062 202 describes a structure in which the emulsion layers are sandwiched between two coupler containing layers.
Japanese Kokai Patent Application No. Sho 53[1978]-65730 teaches using an additional 0.01-0.3 g/m.sup.2 of yellow coupler in the interlayer between the blue light sensitive layer and the green light sensitive layer.